Reversible Actuation via Photoisomerization-Induced Melting of a Semicrystalline Poly(Azobenzene)

Abstract

Photoisomerization of azobenzene in polymer matrices is a powerful method to convert photon energy into mechanical work. While most previous studies have focused on incorporating azobenzene within amorphous or liquid crystalline materials, the limited extents of molecular ordering and correspondingly modest enthalpy changes upon switching in such systems has limited the achievable energy densities. In this work, we introduce a semi-crystalline main-chain poly(azobenzene) where photoisomerization is capable of reversibly triggering melting and re-crystallization under essentially isothermal conditions. These materials can be drawn into aligned fibers, yielding optically-driven two-way shape memory actuators capable of reversible bending. Scheme 1. Preparation of semi-crystalline poly(azobenzene)s ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Main-chain liquid crystalline polymers bearing periodically grafted folding elements

“…12,13 Inclusion of flexible spacers in the polymer backbone, on the other hand, leads to completely different organization of the chains in the mesophase, which is also of a smectic-type; however, the chains typically lay extended perpendicular to the layer plane, with the mesogenic units lying either perpendicular (smectic A) or at an angle (semctic C) to the plane. A variety of flexible spacers, such as polymethylene, [17][18][19][20][21][22][23][24][25][26][27] oligoethylene oxide, [28][29][30][31][32][33] and oligosiloxane segments [34][35][36] have been incorporated within the backbone of the main-chain LCPs; oligosiloxane spacers have been found to reduce the phase transition temperatures most drastically and often lead to the formation of a nematic mesophase with wide temperature stability. In the cases where the pendant substituent is installed on the flexible spacer of the backbone, folding of the chain to exclude the pendant substituent is often seen; [37][38][39][40][41][42][43] this happens especially when a bulky pendant unit, such as a ferrocenyl 38,39 or phenyl unit, [40][41][42][43] is present.…”

Photochemical-induced phase transitions in photoactive semicrystalline polymers

“…Following Kuenstler et al [13], the molecular structure of the photoactive semi-crystalline polymer is illustrated in Fig. 1a.…”

“…1a) under illumination of certain wavelength. Experiment shows that there are three phases: trans-(semi)crystal, cis 12/1/2020 melt and trans melt [13]. Illumination of the trans-semicrystalline phase and the consequent photoisomerization can lead to a light-induced melting from the trans-semicrystalline phase to the cis-melt phase.…”

“…A new class of materials, photoactive semi-crystalline polymers, have recently been developed [13] which exploit the light-induced crystallization and melting of photoactive crystals and polymers due to photoreaction [14][15][16][17][18]. The semi-crystalline to melt transition temperature of polymers of photoactive molecules like poly-azobenzene depends on the light-induced trans to cis ratio.…”

Photo‐Responsive Behavior of Azobenzene Based Polar Hockey‐Stick‐Shaped Liquid Crystals

“…An interesting photo‐orientation phenomenon has been explored in films of azobenzene containing bent‐shaped compounds [38,39] . Owing to the isomerization, light‐induced phase transitions i. e. from solid to liquid, [40–42] and gel to sol [43] have been enormously discussed in azobenzene based materials. The surface topographies of amorphous, nematic, as well as crystalline films of an azobenzene‐containing bent‐core compound has been investigated via polarizing optical microscopy and atomic force microscopy [44] .…”